Vibrational Raman and infrared spectroscopy are used to probe the dynamical, conformational, functional and thermodynamic properties of both model and intact membrane assemblies. Emphasis is placed on elucidating lipid-lipid and lipid-protein interactions within the bilayer aggregate. For example, both Fourier-Transform (FT) Raman and visible laser Raman techniques were employed in elucidating the thermotropic and acyl chain packing characteristics of model phosphatidylcholine bilayer assemblies whose variation in sn-2 chain unsaturation mimics naturally occurring lipids in biological membranes. The noncooperative phase transition behavior for the multilaimellar lipid systems containing four and six double bonds in the unsaturated chain (namely, PAPC and PDPC) was interpreted in terms of specific acyl chain clusters. That is, microdomains are formed in which the lipid molecules pack in a manner that maximizes primarily the Van der Waals interactions between saturated hydrocarbon sn-1 chains. A regulation of acyl chain unsaturation, and hence the degree of microdomain heterogeneity, provides a mechanism by which organisms can control the bilayer properties responsible for optimizing the various membrane functions associated with integral proteins.